The invention relates to a compressor installation with an injection-cooled compressor, and a method for compressing a gas in a compressor installation.
Compressors and in particular screw-type compressors are used to compress gaseous media, in particular air sucked in from the atmosphere. Generally, it is intended to provide the gas with a maximum pressure level, at a moderate temperature and free of impurities.
In order to provide the gas at a low temperature at the pressure outlet of a screw-type compressor, different liquid injection pumps have been developed for screw-type compressors. The coolant used for such injection-cooled screw-type compressors is oil, water or a chemical coolant mixture.
It is well known that with screw-type compressors, a low gas outlet temperature is generally coupled to a high efficiency. However, it is no longer tolerable in these days to trade in a good energy efficiency for a strain on the environment by hazardous substances or an excessive consumption of water.
CH-A-564 153 describes a screw-type compressor with a closed pressure cooling circuit in which oil circulates. From CH-A-564 153 it is known that the size of oil droplets and their trajectories have a substantial influence on the cooling effect Further, it is known that a fine atomization of oil droplets provides for good cooling, yet very small oil droplets make it difficult to separate the oil later on and will cause impurities in the pressure gas. The lubricant oil meant for lubrication and cooling is injected into the screw-type compressor and limits the heat-up of the gas to be compressed. Together with the compressed gas, it is led off into a precipitation vessel where the lubricant oil carried along in the pressure gas will precipitate. From the precipitation vessel, the pressure gas is led out of the compressor installation via a cooling means. The lubricant oil is returned to the compressor via a coolant conduit and an oil cooling means and it is again mixed with the gas to be compressed in the compressor. This document does address the suitability of water, yet offers no concrete hint as to a corresponding embodiment.
Using oil as the coolant and lubricant means a burden to the environment since it is impossible to clear all of the lubricant oil from the pressure gas. Moreover, such a screw-type compressor installation cannot be used in applications where a pressure gas entirely free of oil is necessary.
EP-B-0 389 036, DE-A-40 42 177 and EP-B-0 258 255 disclose concrete examples for using water as the coolant liquid. Using water as the coolant allows to avoid unfavorable impurities in the pressure gas. The presence of water itself in the pressure gas poses no problems for most applications, in particular when the pressure gas is air sucked in from the atmosphere and compressed in the compressor.
Generally, generating pressure gas goes together with a high consumption of energy. For this reason, efforts were made to reduce the energy consumption of generating pressure gas by influencing the march of temperature inside the screw-type compressor.
In the screw-type compressor of EP-B-0 389 036, the fluid is injected directly into the compressor housing in an effort to lower the energy consumption when generating pressure gas. A part of the cooling water to be injected is injected into the take-in side of the compressor, while the major part of the cooling water is injected into the compression space of the screw-type compressor at various locations with respect to the pressure increase.
According to DE-A-40 42 177, the temperature in the working chamber is intended to be reduced by providing atomizer nozzles that produce a fine atomized mist Further, these atomizer nozzles are supposed to effect a more uniform march of temperature in the working chamber.
The energy consumption of a screw-type compressor is substantially determined by the number of rotations of the rotors. The acceleration of the pressure gas particles during compression, which is a finction of the number of rotations, is an important factor for the braking of the rotors and, thus, for the energy consumption of a screw-type compressor. Moreover, the number of rotations of the rotors influences the friction of the bearings and the mutual friction of the rotors. Lower speeds allow for a reduction of the losses that are a function of the rotational speeds. On the other hand, leaks between the rotors and between the rotors and the compressor housing will have particularly negative effects with low speeds, since backflow losses prevent obtaining the intended pressure level and high rates of pressure gas.
In order to minimize such backflow losses, EP-B-0 258 255 suggests to produce a film on the rotors and the inner wall of the housing by injecting water. To this end, a quantity of water is supposed to be injected that is about four times greater than the quantity necessary for a complete saturation of the pressure gas. The cooling water is to be injected into the take-in channel or into the compressor housing. During compression, the gas to be compressed will heat up and a part of the injected water is vaporized in the process. The water not vaporized seals the rotors of the compressor and the housing against each other. Nevertheless, the rotors are intended to rotate at tip circle speeds corresponding to those of dry-running rotors.
Together with the compressed gas, also the cooling water is led out from the screw-type compressor. The mixture of gas and cooling water is led via a cooling means to a separator where gas and liquid cooling water are separated. The separated cooling water is led to a pressureless reservoir from where it may again be fed to the screw-type compressor by means of a pump. However, only a part of the injected water is regained. When the water injection is set to a minimum value, there is no return of water at all and the entire quantity of water needed is supplied from fresh water sources.
U.S. Pat. No. 4,968,231 describes a compressor installation with a closed pressure coolant circuit in which an aqueous solution of potassium borate circulates for lubrication and cooling. Downstream of the compressor, a settling vessel is arranged in the coolant circuit, the vessel being a pressure container that is connected with the outlet of the compressor in a pressure-tight manner. The pressure prevailing in the pressure container urges the borate solution circulating in the compressor installation towards the compressor via a filter and a cooling means. The borate solution injected into the compressor is heated by the compressed gas and reaches the settling vessel together with the pressure gas also heated up. Again, also the compressor installation known from U.S. Pat. No. 4,968,231 is harmful to the environment, by releasing borate and by high energy consumption.
Some known screw-type compressors featuring the injection of liquid into the compressor housing have already attained a favorable level with regard to the energy consumption. Yet, as before, atomizing the coolant is difficult since the evaporation of water causes mineral deposits inside the compressor, which cause large tolerances and, in the course, large gaps as well as deteriorated efficiency.